Fortress Forever was a community built Team Fortress mod intended to serve as an unofficial sequel to the popular shooter. At the time, the Team Fortress community had been waiting nearly ten years for Valve's sequel, prompting the mod community to take their own action.
There was a problem, though: after all that effort, Valve ended up releasing Team Fortress 2 on October 9, 2007. That's less than a month after Fortress Forever rolled out. While Fortress Forever did win PC Gamer's 'Mod of the Year' award in 2007, it inevitably languished once Valve's official sequel became available.
Now it has a new lease on life. Fortress Forever has appeared on Steam as a free-to-play title, so if you were disappointed by the slower pace of Team Fortress 2 or just want a super fast shooter it's well worth checking out. Better still, it has its own dedicated servers, which you can read about over on the official Fortress Forever forum.